Optical fiber sensors have a wide range of applications in aerospace, energy, electric power, transportation, and communications because of their advantage such as anti-electromagnetic interference, small size, and low weight. Traditional temperature sensing technologies based on fiber gratings, optical microcavities, Fabry-Perot interferometers, and Mach-Zehnder interferometers cannot achieve distributed measurements. Distributed fiber optic sensing technologies based on Raman scattering are incapable of achieving long-distance sensing. Therefore, a large amount of research has focused on Brillouin-scattering-based fiber sensing. Brillouin optical correlation-domain reflectometry (BOCDR) based on external modulation, can achieve distributed sensing with high spatial resolution and solve the problems of measuring spatial resolution fluctuations and laser sources that require high-frequency and large-amplitude modulation in BOCDR based on internal modulation. However, a spectrum aliasing problem in BOCDR based on external modulation affects the sensing system’s spatial resolution. In this study, we report an external modulation BOCDR system with a large modulation center frequency to overcome signal noise aliasing and achieve temperature sensing with a high spatial resolution.
In this study, we first constructed a light source system for BOCDR based on external modulation, which consists of a function generator, a voltage-controlled oscillator, an electro-optic modulator, a laser source, an erbium-doped fiber amplifier, and an optical bandpass filter. During the measurement process, the Brillouin gain spectrum was obtained by continuously changing the modulation frequency, and the temperature information was demodulated from the measurement results. The different frequency components of the reference and Stokes light in the BOCDR system based on external modulation were compared (Fig. 2), and the impact of beat spectrum noise was analyzed. The modulation center frequency influences the sensing system’s spatial resolution and dynamic measurement range. Subsequently, a novel external modulation BOCDR based on a large modulation center frequency was proposed (Fig. 3). For example, the modulation center frequency was set higher than 14.3 GHz to eliminate the impact of beat spectrum noise and obtain a spatial resolution of 15 cm.
At a room temperature of approximately 22 ℃, a 0.6 m length heating section in 17.2 m sensing fiber was heated to 25, 30, 40, 50, and 60 ℃, and the Brillouin frequency shifts (BFSs) of the heated section corresponding to five different temperatures were measured. By linear fitting, the BFS temperature coefficient of the fiber under test is 1.15 MHz/°C (Fig. 5). In a verification experiment of temperature measurement, the temperature of the 0.6 m-length fiber was heated to 46.8 ℃ by using a water bath, and the distribution of the Brillouin gain spectrum was measured (Fig. 6). Lorentz fitting was performed to extract the BFS (Fig. 7) after subtracting the lowest noise and normalizing the results. From the BFS distribution of the sensing fiber, the spatial resolution of the system is calculated as 11.6 cm, and the measured result of the heated fiber is (46.64±0.17) ℃. When the 0.6 m length fiber is heated to 57.7 ℃, the measured result is (57.57±0.26) ℃. Compared with the sensing performances of four reported typical BOCDR systems, the proposed BOCDR system has a high spatial resolution and low measurement uncertainty. The measurement uncertainty of the proposed system is one order of magnitude lower than that in the other four reported BOCDR systems (Table 1). Therefore, the proposed BOCDR system with a significant modulation center frequency can achieve temperature-distributed sensing with high spatial resolution and measurement accuracy.
This study proposes a new BOCDR based on external modulation with a large modulation center frequency, which can suppress the spectrum aliasing of signals and achieve distributed temperature sensing with a high spatial resolution. The proposed method has a high spatial resolution, and its measurement uncertainty is one order of magnitude lower than similar methods. The causes of beat noise in BOCDR based on external modulation and its impact on the Brillouin scattering signal were analyzed. According to the measurement range and spatial resolution requirements in the BOCDR system based on external modulation, the magnitude of the modulation center frequency is calculated and determined. A larger modulation center frequency can suppress the aliasing effect of beat spectrum noise on the Brillouin scattering signal, resulting in a higher spatial resolution and larger temperature measurement range. In a verification experiment, we successfully implemented temperature-distributed sensing with a spatial resolution of 11.6 cm on a 17.2 m long G657 optical fiber, with an uncertainty of 0.26 ℃ for temperature measurement.
